1. Technical Field
The present invention relates in general to perpendicular magnetic recording in disk drives and, in particular, to an improved system, method, and apparatus for detecting signal-to-noise ratio decay in perpendicular magnetic recording.
2. Description of the Related Art
Thermal decay due to the “superparamagnetic effect” in longitudinal magnetic recording is becoming a significant concern as the rate of areal density increases rapidly. In the past, substantial effort was devoted to characterizing media thermal decay and understanding the impact of media decay on the ultimate recording performance. Precise and quantitative prediction of media thermal decay lifetime was commonly believed to be an important task. In the prior art, amplitude or magnetization versus time was measured, then the decay rate was determined in percentage decrease per decade of log(time).
Perpendicular magnetic recording (PMR) has been investigated as a way to extend beyond the “superparamagnetic limit” of conventional longitudinal recording. Hard disk drive (HDD) companies are intensively engaged in PMR development. Generic architectures of longitudinal recording 11 and perpendicular recording 13 are depicted in FIG. 1. An important feature of PMR is the “deep gap field” generated by the single pole writer and the soft magnetic underlayer (SUL) in the recording media. However, there are two concerns with PMR. The demagnetization field is generally larger in the PMR, which may induce an increase in background noise over time. Secondly, the SUL is part of the writer during the writing process, but is embedded in the PMR media. SUL is a soft magnetic material (e.g., NiFe permealloy) and easily forms a variety of complex domain structures. The domain movement over time and temperature in the SUL may generate adverse noise evolution. The characterization of these noise dynamics is important to successfully launch PMR HDD products.
The article, An Experimental Study of the Effect of Thermal Decay on Noise and Nonlinear Distortions in Perpendicular Media, by W. Zhu, H. Zhou, and J. Judy (IEEE Magn. 40, No. 4, p. 2610–2612 (2004)), provides details of writing a pseudo-random sequence on perpendicular magnetic media and using an oscilloscope to monitor nonlinear distortion and its related noise background for addressing SNR decay. That method was employed in longitudinal magnetic recording in the past and, thus, is merely an example of applying the same method to perpendicular media without any improvements. Unfortunately, that method cannot be implemented in any PMR disk media manufacture testing or at an early stage of PMR media development due to its complexity, the difficulty of capturing very small signal distortion signals, and the difficulty of accurately monitoring an evolution of small distortion as a function of time.
U.S. Patent Application No. 2003/0016461, provides a method of determining a time domain equalized, signal-to-total distortion ratio and an equalized signal-to-noise ratio via writing a pseudo-random 127-bit pattern on a magnetic media. That disclosure determines thermal characterization in HDDs and is not applicable to component level testing, such as magnetic disk media screening. It also requires a channel IC chip for waveform equalization, and needs several specific analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC) units for manipulation. It is impossible to implement that design on conventional spin stand magnetic testers. Thus, one skilled in the art would not employ that prior art method for routine PMR disk media testing and screening. Finally, U.S. Pat. No. 6,630,824, and U.S. Patent Application No. 2002/0063559, provide performance evaluation methods for signal decay, but not SNR decay.
Thus, although the prior art measures signal decay in magnetic recording systems, it is unable to analyze the noise-induced instability in PMR systems. It would be desirable to develop a comprehensive method to resolve this issue. In the present disclosure, a solution is presented for detecting signal decay and noise evolution at the same time.